Vertically aligned mode liquid crystal display

ABSTRACT

A liquid crystal display is provided, which includes a first insulating substrate; a pixel electrode formed on the first substrate and having a first aperture pattern; a thin film transistor formed on the first substrate and switching voltages applied to the pixel electrode; a second insulating substrate opposite to the first substrate; a reference electrode formed on the second substrate and having a second aperture pattern to partition the pixel electrode into a plurality of subareas together with the first aperture pattern; and a liquid crystal layer interposed between the first substrate and the second substrate and having the thickness ranging 3.4-4.0 μm. The above described adjustment of the cell gap and the applied electric field can keep the response time of an LCD to be equal to or lower than a predetermined value.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a liquid crystal display, and inparticular to a vertically aligned liquid crystal display to implement awide viewing angle by dividing a pixel region into a plurality of smalldomains using domain dividing means.

(b) Description of the Related Art

Generally, a liquid crystal display (“LCD”), which includes an upperpanel with a common electrode and color filters, a lower panel with thinfilm transistors (“TFTs”) and pixel electrodes, and a liquid crystallayer disposed therebetween, is a display device that displays images byapplying different electric potentials to the pixel electrodes and thecommon electrode to generate electric field to change the arrangement ofliquid crystal molecules, thereby controlling the transmittance oflight.

Meanwhile, an LCD has a major disadvantage of narrow viewing angle. Toovercome the disadvantage, several methods for increasing the viewingangle have been developed. Among them, a method is promising, whichaligns liquid crystal molecules vertically with respect to upper andlower panels and provides aperture patterns or protuberances on pixelelectrodes and a common electrode opposite thereto.

The method of forming aperture patterns is to form aperture patternsboth on the pixel electrodes and the common electrode and to adjust tiltdirections of the liquid crystal molecules using fringe field generateddue to the aperture patterns hereinafter, referred to as “PVA (patternedvertically aligned) mode”).

Meanwhile, an LCD has a disadvantage of slow response compared with CRTbecause it tales time to change the arrangement of the liquid crystalmolecule when applied with driving voltages. When the response speed islower than a given value, the high image quality in displaying movingpictures cannot be not obtained since after-images are recognized.Consequently, methods for increasing the response speed as much aspossible have to be studied.

SUMMARY OF THE INVENTION

An object of the present invention is to improve the response speed ofthe vertically aligned mode LCD.

To accomplish the object, the present invention keeps the cell gap ofthe vertically aligned mode LCD in a predetermined range.

In detail, a liquid crystal display is provided, which includes a firstinsulating substrate; a pixel electrode formed on the first substrateand having a first aperture pattern; a thin film transistor formed onthe first substrate and switching voltages applied to the pixelelectrode; a second insulating substrate opposite to the firstsubstrate; a reference electrode formed on the second substrate andhaving a second aperture pattern to partition the pixel electrode into aplurality of subareas together with the first aperture pattern; and aliquid crystal layer interposed between the first substrate and thesecond substrate and having the thickness ranging 3.4-4.0 μm.

It is preferable that liquid crystal molecules contained in the liquidcrystal layer are aligned substantially perpendicular to the firstsubstrate and the second substrate in absence of electric field, and thestrength of electric field applied to liquid crystal molecules containedin the liquid crystal layer is in a range of 1.17 V/μm-1.33 V/μm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an LCD according to an embodiment of thepresent invention;

FIG. 2 is a graph illustrating the on and off response times as functionof the cell gap in an LCD according to an embodiment of the presentinvention;

FIG. 3 is a graph showing the minimum response time as functions of thecell gap in an LCD according to an embodiment of the present invention;

FIG. 4 is a graph of the response time as function of white grayvoltages for various cell gaps in an LCD according to an embodiment ofthe present invention;

FIG. 5 is a graph showing voltages giving the minimum response time asfunction of cell gap in an LCD according to an embodiment of the presentinvention; and

FIG. 6 is a graph showing electric fields giving the minimum responsetime as function of an LCD according to an embodiment of the presentinvention. 110: TFT array panel 3: liquid crystal layer 123: gateelectrode 140: gate insulating layer 151: polysilicon layer 171: dataline 173: source electrode 175: drain electrode 180: passivation layer190: pixel electrode 210: color filter panel 230: color filter 220:black matrix 250: overcoat 270: reference electrode 191 and 271:aperture

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Now, an LCD according to an embodiment of the present invention will bedescribed with reference to the drawings.

FIG. 1 is a sectional view of an LCD according to an embodiment of thepresent invention.

First, a TFT array panel will be described.

A gate wire is formed on an insulating substrate 110. The gate wireincludes a gate line (not shown) extending in a transverse direction, agate pad (not shown) connected to one end of the gate line to receivegate signals from an external device and to transmit them to the gateline and a gate electrodes 123 of a TFT, which is a portion of the gateline.

The gate wire may be formed of a single layer, double layers or triplelayers. The single layer is preferably made of Al or Al—Nd alloy, andthe double layers preferably include a lower layer made of materialhaving good physical-chemical characteristics such as Cr, Mo or Mo alloyand an upper layer made of material having low resistivity such as Al orAl alloy.

A gate insulating layer preferably made of SiNx is formed on the gatewire.

A semiconductor layer 151 made of semiconductor such as hydrogenatedamorphous silicon is formed on the gate insulating layer. Thesemiconductor layer 151 overlaps the gate electrode 123.

A contact layer 163 and 165 made of material such as n+ hydrogenatedamorphous silicon heavily doped with n-type impurity is formed on thesemiconductor layer 151. The contact layer 163 and 165 is divided intotwo portions opposite each other with respect to the gate electrode 123.

A data wire is formed on the contact layer 163 and 165. The data wireincludes a source electrode 173 formed on a source portion 163 of thecontact layer, a data line 171 connected to the source electrode 173 andextending in a longitudinal direction, a data pad (not shown) connectedto one end of the data line 171 and applied with image signals from anexternal device, and a drain electrode 175 formed on a drain portion 165of the contact layer opposite the source electrode 173 with respect tothe gate electrode 123.

The data wire may have a single-layered structure, a double-layeredstructure or a triple-layered structure like the gate wire. Thesingle-layered structure is preferably made of Al or Al—Nd alloy, andthe double-layered structure preferably include a lower layer made ofmaterial having good physical-chemical characteristics such as Cr, Mo orMo alloy and an upper layer made of material having low resistivity suchas Al or Al alloy.

A passivation layer 180 is formed on the data wire. The passivationlayer 180 covers and protects a channel portion between the sourceelectrode 173 and the drain electrode 175, and, in this embodiment, itcovers all the data wire as well as the channel portion, except for thecontact hole 181 exposing the drain electrode 175 and a contact hole(not shown) exposing the data pad.

A pixel electrode 190 made of a transparent conducting material such asITO (indium tin oxide) or IZO (indium zinc oxide) is formed on thepassivation layer 180, and a plurality of auxiliary pads (not shown)made of the same material as the pixel electrode 190 are formed on thegate pad, a storage electrode pad and the data pad. In a reflective typeLCD, the pixel electrode 190 is made of metal reflecting light well suchas Al.

The pixel electrode 190 has an aperture pattern 191.

Next, a color filter panel will be described.

A black matrix made of one of a single chromium layer, double layers ofchromium and chromium oxide, and organic material containing blackpigments is formed on a transparent insulating substrate 210. Aplurality of red, green and blue color filters are formed on the blackmatrix. The red, green and blue color filters 230 are assigned in eachpixel area partitioned by the black matrix 220. An overcoat 250 made oforganic insulating material is formed on the color filters 230, and areference electrode 270 made of transparent conductive material isformed on the overcoat. An aperture pattern 271 is provided in thereference electrode 270. The function of the overcoat 250 is to preventthe color filters from being exposed through the aperture pattern.

The LCD according to the first embodiment is obtained by aligning andcombining the TFT array panel and the color filter panel and theninjecting liquid crystal material 3 therebetween. The liquid crystalmolecules contained in the liquid crystal material are aligned such thattheir director is perpendicular to the substrates 110 and 210 in absenceof electric field between the pixel electrode 190 and the referenceelectrode 270. The TFT array panel and the color filter panel arealigned so that the pixel electrode 190 exactly corresponds to the colorfilter 230. In this way, a pixel region is divided into a plurality ofsmall domains by the aperture patterns 191 and 271. The classificationsof the small domains are determined based on the tilt directions of thedirector of the liquid crystal molecules.

In this LCD, the cell gap d, which is defined as the thickness of theliquid crystal layer, keeps in a range of 3.4-4.0 μm. This enables theresponse time to be generally equal to or lower than 25 ms, which is astandard response time of LCD products displaying moving pictures.

For the strength of the electric field in a range 1.17-1.33 V/microns,the response time can be equal to or lower than 25 ms, which is astandard response time of LCD products displaying moving pictures.

Now, the reason for the above determined ranges of the cell gap d andthe electric field as above will be examined.

Generally, the response time of an LCD is known as proportional to thesquare of the cell gap, and a TN (twisted nematic) mode LCD and a CE(coplanar electrode) mode LCD conform to this rule. However, a PVA modeLCD often does not conform to this rule. Accordingly, a response time asfunction of a cell gap is required to be measured in order to find thecell gap providing the fastest response speed.

Table 1 illustrates a measured response time as function of cell gap ina PVA mode LCD according to an embodiment of the present invention. Theunit of the response time is milliseconds. TABLE 1 Cell gap [μm] 2.8503.155 3.576 3.695 3.923 3.936 4.132 ON [ms] 40.21 26.59 16.63 15.3216.32 14.05 13.76 OFF [ms] 4.75 5.23 6.95 7.35 8.00 10.36 11.98 ON + OFF[ms] 44.96 31.82 23.58 22.68 24.33 24.11 25.75

FIG. 2 shows curves corresponding to Table 1.

As shown in FIG. 2, it can be seen that, in the PVA mode, when the cellgap is lower than about 3.66 μm, the on response tine ON rapidlyincreases as the cell gap becomes smaller. When the cell gap is largerthan about 3.66 μm, the on response time ON also increases as the cellgap becomes smaller, even though the increasing gradient is vary smooth.In contrast, the off response time OFF increases nearly in proportion tothe square of the cell gap. The curve ON+OFF represents the sum of theon response time ON and the off response time OFF. The curve ON+OFF forthe cell gap lower than 3.66 μm varies mainly dependent on the onresponse time ON, while the curve ON+OFF for the cell gap larger than3.66 μm varies mainly depending on the off response time OFF. Thus, thesum ON+OFF of the on response tine ON and the off response time OFFexhibits the minimum at about 3.66 μm and tends to increase as the cellgap increases or decreases with respect to 3.66 μm.

FIG. 3 is a graph showing minimum response time as function of cell gapin an LCD according to an embodiment of the present invention.

As shown in FIG. 3, the response time for the cell gap of about 3.66 μmis about 21.37 μm. In addition, it can seen that the range of the cellgap, which meets the standard response time equal to or lower than 25 msof LCD products, is 3.40-4.00 μm.

Next, the strength of electric field will be examined.

Since the behaviors of liquid crystal molecules become faster forincreasing strength of electric field, the response time is mistakenlyconsidered to become shorter, but it is not true in the actual world.This is because the over-strong electric field strengthens bad, flow oftexture from the data wire to more than a given extent to delay thebehaviors of the liquid crystal molecules, thereby making the responsetime for displaying desired images to become rather larger. The textureis referred to a phenomenon that some liquid crystal molecules near theapertures of the pixel electrode exhibit abnormal behaviors to tilt inunexpected directions due to distortion of the electric filed near theapertures of the pixel electrode. The back flow is referred to aphenomenon that such a texture moves in a direction opposite theexpected direction in which the liquid crystal molecules are expected tomove, due to a strong initial electric field. The region with the backflow occupies only a portion of the entire pixel region, and the flowreturns to the expected direction within a few seconds. Accordingly, itis required to find out the strength of the electric field, whichminimizes the back flow.

Table 2 is a graph showing the response time versus white gray voltagefor various cell gaps in an LCD according to an embodiment of thepresent invention. TABLE 2 Cell gap Response time per white voltages 2.85 μm Voltages 2.64 2.80 3.08 3.40 3.96 5.04 ON 41.97 34.90 31.9030.25 32.76 40.21 OFF 4.36 4.03 4.01 4.11 4.56 4.75 ON + OFF 46.33 38.9435.91 34.35 37.31 44.96 3.155 μm Voltages 2.60 2.80 3.04 3.40 3.96 5.04ON 45.89 38.57 30.68 27.36 27.91 26.59 OFF 4.63 4.63 4.76 4.89 5.11 5.23ON + OFF 50.52 43.20 35.43 32.25 33.02 31.82 3.576 μm Voltages 2.60 2.763.00 3.40 3.90 5.04 ON 56.83 45.25 37.14 28.63 26.74 16.63 OFF 5.56 5.455.83 5.80 6.32 6.95 ON + OFF 62.39 50.71 42.97 34.43 33.06 23.58 3.695μm Voltages 2.60 2.80 3.04 3.36 3.84 5.08 ON 58.95 48.25 37.51 28.7025.63 15.32 OFF 6.35 5.90 5.89 6.53 6.77 7.35 ON + OFF 65.30 54.15 43.4035.22 32.40 22.68 3.923 μm Voltages 2.52 2.72 2.96 3.28 3.86 5.00 ON66.62 52.60 41.21 35.90 28.45 16.32 OFF 6.32 6.12 6.27 6.96 7.35 8.00ON + OFF 72.94 58.72 47.48 42.86 35.80 24.33

FIG. 4 shows curves corresponding to Table 1.

As shown in FIG. 4, the minimum response times for respective cell gapsare shown at the different specific voltages, and the response timesincrease as the applied voltages become larger or smaller than thespecific voltages. The voltage is referred to a potential differencebetween the pixel electrode and the reference electrode.

The minimum response times for respective cell gaps are shown in Table3. TABLE 3 Voltage [V] Electric Field [V/μm] for Minimum Response forMinimum Response d [μm] Time Response Time Time [ms] 2.850 3.77 1.3234.48 3.155 4.27 1.35 28.65 3.576 4.70 1.31 23.61 3.695 4.65 1.26 21.673.923 4.74 1.21 24.72

FIG. 5 shows a curve of the voltages giving the minimum response time asfunction of cell gap described in Table 3. In addition, FIG. 6 shows acurve of the electric field giving the minimum response time as functionof cell gap described in Table 3.

As can seen above, when the cell gap d, which is the thickness of theliquid crystal layer, is in a range of 3.4-4.0 μm and the strength ofthe electric field is in a range of 1.17-1.33 V/μm, as shown in FIG. 6,the response time becomes equal to or lower than 25 ms, which is thestandard response time of LCD products displaying moving pictures.

Although preferred embodiments of the present invention have beendescribed in detail hereinabove, it should be clearly understood thatmany variations and/or modifications of the basic inventive conceptsherein taught which may appear to those skilled in the present art willstill fall within the spirit and scope of the present invention, asdefined in the appended claims. Especially, the arrangement of theapertures formed on the pixel electrodes and the reference electrodesmay have a variety of modifications.

As described above, the adjustment the cell gap and the applied electricfield can keep the response time of an LCD to be lower than apredetermined value.

1. A liquid crystal display comprising: a first insulating substrate; apixel electrode formed on the first substrate and having a firstaperture pattern; a thin film transistor formed on the first substrateand switching voltages applied to the pixel electrode; a secondinsulating substrate opposite to the first substrate; a referenceelectrode formed on the second substrate and having a second aperturepattern to partition the pixel electrode into a plurality of subareastogether with the first aperture pattern; and a liquid crystal layerinterposed between the first substrate and the second substrate andhaving thickness in a range of 3.4-4.0 μm.
 2. The liquid crystal displayof claim 1, wherein liquid crystal molecules contained in the liquidcrystal layer are aligned substantially perpendicular to the firstsubstrate and the second substrate in absence of electric field.
 3. Theliquid crystal display of claim 1, wherein strength of electric fieldapplied to liquid crystal molecules contained in the liquid crystallayer is in a range of 1.17 V/μm-1.33 V/μm.